The present invention relates to a motor drive method for an industrial robot which is capable of preventing suspension of operation of the industrial robot, and deviation of the track of movement from a command track, attributable to an overload state of a motor.
An industrial robot generally comprises a plurality of operating sections (e.g., joints of an arm section) severally driven with respect to one or more axes (e.g., around an axis of rotation and along an axis of movement), and a plurality of motors as actuators for severally driving corresponding ones of the operating sections with respect to corresponding axes. The individual motors are driven so that an end position or a final-stage operating section (e.g., wrist section), as an object of speed control, moves at a command speed along a command track. Typically, in a servo system provided for a respective axis of each operating section, pulses (corresponding to a move command value P per unit time for the axis concerned) determined in accordance with a command position and the command speed for the finalstage operating section, are distributed, while a speed feedback signal VF (indicative of the rotational speed of a motor M) is integrated at numeral 103 to obtain a position signal feedback signal Pf, as shown in FIG. 1. Then, a speed command V, which is equivalent to the product of a position loop gain Gp designated by numeral 100 and a deviation Er between the two signals P and Pf, is compared to the speed feedback signal Vf. Subsequently, a torque command value T, which is equivalent to the product of a speed loop gain Gv designated by numeral 101 and the deviation between the two signals V and Vf, is supplied to a servo amplifier 102. A driving current corresponding to the torque command value T is supplied from the servo amplifier 102 to the servomotor M, whereupon the motor produces a required torque. Similar servo systems provided for the individual axes severally operate in the aforementioned manner, thereby working the robot.
If the rotational frequency of the motor M decreases with the increase of load on the motor M, during the operation of the robot, the deviation Er, speed command value V, and torque command value T increase, thereby augmenting the output torque of the motor M to increase the motor speed. Meanwhile, a torque limiter 104 is provided for preventing an overcurrent from flowing through the motor M, and a maximum allowable torque value is previously set in the torque limiter. If the increased torque command value T exceeds the maximum allowable torque value, the motor driving current supplied from the servo amplifier 102 is limited to a value corresponding to the maximum allowable torque value. Consequently, the motor torque output cannot increase to the value corresponding to the increase of the load, so that the deviation Er increases. When the deviation Er attains a predetermined value, the operation of the robot is stopped. If the deviation Er does not attain the predetermined value, the actual rotational speed Vf of the motor does not follow up the speed command value V, although the operation of the robot continues without being stopped. Accordingly, the operating speed of that operating section of the robot which is associated with the axis subject to the deviation increase ceases to follow up the command value. On the other hand, the follow-up performance will not be deteriorated with respect to at least most of the other axes of the operating section concerned and the axes of the other operating sections. Thus, there may be an operative misalignment between the axes of one operating section or between the operating sections, so that the move track of movement of the final-stage operating section, as the object of end position and speed control, will deviate from the command track.